Philosophers such as Ludwig Wittgenstein, W. V. Quine, and Jacques Derrida have questioned the possibility of such a rigorous study of language by questioning many of the assumptions necessary for such a study, and have put forth their own views on the nature of language. There is no end in sight to this debate.

Some like to make parallels to biology, where it is not always possible to make a well-defined distinction between one species and the next. In either case, the ultimate difficulty may stem from the interactions between languages and populations. (See Dialect or August Schleicher for a longer discussion.)

Scientists do not yet agree on when language was first used by humans (or their ancestors). Estimates range from about two millionyears ago, during the time of Homo habilis, to as recently as forty thousand years ago, during the time of Cro-Magnon man.

One prominent artificial language, called Esperanto, was created by L. L. Zamenhof. It was a compilation of various elements of different languages, which was intended to be an easy-to-learn language.

Other writers, such as J. R. R. Tolkien, have created fantasy languages, for literary, linguistic, or personal reasons. One of Tolkien's languages is called Quenya, which is a form of Elvish. It includes its own alphabet and its phonology and syntax are modelled after Finnish. The writer, linguist and Star Trek actor James Doohan devised the original vocabularies of Vulcan and Klingon speech which have been developed by others into full languages.

While the term animal languages is widely used, most researchers agree that they are not as complex or expressive as the human language. Some researchers argue that there are significant differences separating human language from the communication of other animals, and that the underlying principles are not related.

Other researchers argue that a continuum exists among the communication methods of all animals. Almost all researchers agree that human language is more complex than communication between other animals.